1. Field of the Invention
The invention relates to a process for producing low sulfur synthesis gas (syngas) from natural gas with C4+/C5+ hydrocarbon recovery. More particularly the invention relates to producing very low sulfur syngas from sour natural gas with C4+/C5+ hydrocarbon recovery and to the use of the syngas for hydrocarbon synthesis. The sulfur content of the syngas is less than 10 vppb and preferably less than 3 vppb. The process includes treating the natural gas by amine scrubbing, low temperature hydrocarbon and sulfur separation, followed by contact with zinc oxide and then nickel.
2. Background of the Invention
Hydrocarbon synthesis (HCS) processes are well known and include fixed bed, fluid bed and slurry type processes in which a synthesis gas (syngas) comprising a mixture of H2 and CO is reacted in the presence of a suitable Fischer-Tropsch type of hydrocarbon synthesis catalyst at conditions effective to form hydrocarbons, and preferably paraffinic hydrocarbons which are solid at standard room temperature conditions of temperature and pressure. The syngas is produced by reacting a low molecular weight hydrocarbon gas with oxygen and steam via well known processes which include partial oxidation, catalytic steam reforming and combination thereof, using a fixed or fluid catalyst bed. In a fluid bed syngas generation (FBSG) process, partial oxidation and steam reforming both occur in the presence of the steam reforming catalyst. This process has the advantage of superior heat and mass transfer. In autothermal reforming the hydrocarbon is first partially oxidized and then separately catalytically steam reformed. These and other syngas processes and their relative merits are discussed, for example, in U.S. Pat. Nos. 4,877,550; 4,888,131 and 5,160,456. A preferred source of the low molecular weight hydrocarbon is natural gas in which the hydrocarbon comprises primarily methane with minor amounts (e.g., xcx9c1-10%) of C2+ hydrocarbons, including C4+ hydrocarbons. Other natural gas components include nitrogen, carbon dioxide, water vapor and sulfur in the form of sulfur bearing compounds including H2S, mercaptans (RSH), other organic sulfides generally, carbonyl sulfide (COS) and sometimes minor amounts of carbon disulfide. Sulfur in the feed to a syngas generator will poison the steam reforming catalyst and result in a loss of syngas productivity. Certain HCS catalysts are easily poisoned and permanently deactivated by these sulfur bearing compounds. Those comprising a cobalt catalytic component are particularly sensitive and as little as 0.1 vppm (volume parts per million) of sulfur compounds present in the syngas feed to the HCS reactor will permanently deactivate the catalyst in less than 10 days. Even levels as low as, for example, 10 vppb (volume parts per billion) are unacceptably high for a commercial HCS plant. As the catalyst deactivates, hydrocarbon production decreases and the reactor has to be taken off line for catalyst replacement. Consequently, the ability to achieve highly productive hydrocarbon synthesis with such catalysts, on a sustainable basis, has not yet been achieved. It would be an improvement to the art to be able to produce syngas having less than 10 vppb of sulfur compounds from sour natural gas by a method which also recovers the valuable C4+ and/or C5+ hydrocarbons.
A process for producing low sulfur synthesis gas (syngas) from natural gas which contains C4+/C5+ hydrocarbons, with recovery of these hydrocarbons from the gas, comprises scrubbing or contacting the gas with a liquid sulfur absorbent to remove most of the sulfur, followed by low temperature cooling to remove more sulfur compounds and the C4+/C5+ hydrocarbons, and then contacting the sulfur and hydrocarbon reduced gas first with zinc oxide and then nickel to reduce the sulfur content to less than 0.1 vppb/m (volume parts per million) and preferably less than 80 vppb (parts per billion), before it is passed into the syngas generator. The syngas exiting the syngas generator is then contacted with zinc oxide to remove remaining sulfur from the gas. This process produces a syngas feed having less than 10 vpp, of sulfur in the form of sulfur bearing compounds and recovers the valuable C4+/C5+ hydrocarbons from the feed, so that they are not wasted by being passed into the syngas generator. The C4+/C5+ hydrocarbons recovered from the natural gas are upgraded by hydrorefining and fractionation. The zinc oxide and nickel react with the sulfur compounds remaining in the gas after the scrubbing and cooling, to form zinc sulfide and nickel sulfide. The nickel is preferably nickel metal and in a particularly preferred embodiment the nickel is supported on a support material. Reducing the sulfur content of natural gas fed into an FBSG unit down to less than 0.1 vppm and preferably less than 80 vppb substantially reduces catalyst deactivation in a fluid bed syngas generator and increases the syngas productivity. When the sulfur content in natural gas fed into a FBSG containing a nickel reforming catalyst was reduced to less than 80 vppb, it resulted in less than a 1% per day activity loss. The low sulfur feed comprising primarily methane, is then fed into a syngas generating unit, along with steam and oxygen or air, and preferably oxygen, to produce a syngas comprising a mixture of H2 and CO. The syngas is then contacted with zinc oxide to reduce the sulfur level in the syngas to the less than 10 vppb and preferably less than 3 vppb levels desired for feeding the syngas to a hydrocarbon synthesis (HCS) reactor. It also serves as a guard bed in the event of a sulfur breakthrough upstream of the syngas generator and from sulfur contaminants present in the syngas generating unit and from the other feed components. The very low sulfur syngas is then fed into an (HCS) reactor in which the H2 and CO react in the presence of a suitable Fischer-Tropsch type of hydrocarbon synthesis catalyst at conditions effective to form hydrocarbons. In a slurry HCS process, at least a portion of the synthesized hydrocarbons comprise the slurry liquid and are solid at standard room temperature conditions of temperature and pressure (e.g., 75xc2x0 F. and atmospheric pressure).
In a broad sense, the invention comprises removing sulfur compounds and C4+/C5+ hydrocarbons from natural gas to form a low sulfur gas comprising mostly methane, which contains less than 0.1 vppm and preferably less than 80 vppb of sulfur, by liquid absorption, low temperature separation and contact with zinc oxide followed by nickel. The low sulfur methane gas is then passed into a syngas generating unit to produce a syngas comprising a mixture of H2 and CO having a low sulfur content or used for any other purpose. In a further embodiment, the syngas is contacted with zinc oxide to insure that the level of sulfur in the gas remains at less than 10 vppb and preferably less than 3 vppb of sulfur. By sulfur is meant sulfur compounds which include predominantly H2S and one or more of mercaptans (RSH), other organic compounds generally, carbonyl sulfide (COS) and CS2. Except for the H2S, all of the other sulfur bearing compounds may be considered as organic sulfur compounds.
The liquid absorption or scrubbing may selectively remove only the sulfur compounds or it may remove both the sulfur compounds and CO2, if desired. The choice depends on the amount of CO2 present in the natural gas and the extent to which various CO2 and sulfur compound removal methods, such as amine scrubbing, are feasible. As a practical matter, if the amount of CO2 exceeds about 2 mole % of the gas, removal is necessary to prevent plugging of the low temperature hydrocarbon separation unit downstream of the scrubbing. The use of amine scrubbing for either sulfur or both sulfur and CO2 removal from gas streams, including natural gas, is well known and is disclosed, for example, in U.S. Pat. No. 4,405,585 relating to aqueous solutions of hindered amines. These processes are commercially available from Exxon as their FLEXSORB(copyright) and FLEXSORB PS(copyright) processes. By low temperature separation in the context of the invention is meant cooling the gas down to a temperature of at least 30xc2x0 F., preferably at least 0xc2x0 F. and more preferably at least about minus 20xc2x0 F. This condenses more sulfur compounds out of the gas as well as the C4+/C5+ hydrocarbons. In a preferred embodiment of the invention the gas is cooled down to a temperature of from about 20 to xe2x88x9240xc2x0 F. to maximize C4+/C5+ hydrocarbon recovery. The exact cut point for the C4+ or C5+ hydrocarbons depends on a number of variables, hence C4+/C5+ is used to indicate that at least the C5+ and preferably also C4 hydrocarbons are removed from the gas. It is preferred in the practice of the invention to take advantage of the high pressure (e.g., xe2x89xa71,000 psig) of the natural gas recovered from the gas well, to cool the gas using Joule-Thomson and, if desired, flash expansion. The cooled mixture of gas and liquid C4+/C5+ hydrocarbons is then passed to a gas-liquid separator or knockout drum, to separate the gas from the condensed hydrocarbons. The cooled, sulfur and C4+/C5+ hydrocarbon reduced gas is then passed through one or more guard beds in which it is contacted with one or more sulfur absorbents and/or adsorbents which, in a preferred embodiment of the invention, sequentially comprises first zinc oxide and then nickel, to remove substantially all of the sulfur compounds remaining and produce a gas feed for the syngas generator containing less than 0.1 vppm, and preferably less than 80 vppb of sulfur compounds. It is preferred to heat the cool gas up to a temperature of at least about 150xc2x0 F. and more preferably at least 200xc2x0 F. before it contacts the sulfur absorbents, particularly when contacting the nickel. This increases the capacity and efficiency of the two sulfur absorbents. After passing through the absorbent beds, the gas may be fed into a syngas reactor to produce a syngas feed comprising a mixture of H2 and CO for hydrocarbon syntheses or used for any other purpose.